[CoreCLR] Exclude RCPC2 from ARM64 ReadyToRun defaults

Documentation/docs-mobile/building-apps/build-properties.md

@@ -1903,6 +1903,40 @@ ReadyToRun compilation is composite by default. See
 [`$(MauiEnableFullReadyToRun)`](#mauienablefullreadytorun) to opt in to
 full ReadyToRun.
 
+For ARM64, .NET for Android defaults crossgen2 to:
+
+```text
+--instruction-set:-optimistic,aes,crc,dotprod,lse,rcpc,rdma,sha1,sha2
+```
+
+This disables automatic optimistic instruction-set additions and explicitly
+enables the listed features, excluding RCPC2 without requiring RCPC2 to be
+absent. The listed features become baseline requirements for the precompiled
+image, rather than optional optimistic features. Devices without a required
+feature can reject the image's ReadyToRun code and fall back to JIT compilation.
+
+Other CPU architectures default to `--instruction-set:-optimistic`, which
+disables automatic optimistic instruction-set additions while retaining the
+normal baseline requirements. Neither policy disables hardware-specific
+optimizations in the JIT or tiered compilation.
+
+An explicit `--instruction-set` option in
+`$(PublishReadyToRunCrossgen2ExtraArgs)` overrides this default.
+An instruction-set option in
+`$(PublishReadyToRunCrossgen2CompositeExtraArgs)` also opts the entire
+build out of this default, including assemblies in
+`@(PublishReadyToRunCompositeExclusions)`. The .NET SDK concatenates the
+normal and composite arguments for the composite image, and crossgen2
+does not accept multiple `--instruction-set` options. The opt-out applies
+even when `$(PublishReadyToRunComposite)` is `false`.
+
+To apply an instruction-set policy to both the composite image and
+excluded assemblies, set it in
+`$(PublishReadyToRunCrossgen2ExtraArgs)`, not in the composite-only
+arguments. For example, `--instruction-set:-optimistic` in the normal
+arguments keeps both conservative. Do not specify the option in both
+argument properties.
+
 For more information, see
 [Runtimes and compilation in .NET MAUI][maui-runtimes-compilation].
 

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.CoreCLR.targets

@@ -25,6 +25,18 @@ This file contains the CoreCLR-specific MSBuild logic for .NET for Android.
     <AllowReadyToRunWithoutRuntimeIdentifier Condition=" '$(PublishReadyToRun)' == 'true' and '$(RuntimeIdentifiers)' != '' ">true</AllowReadyToRunWithoutRuntimeIdentifier>
   </PropertyGroup>
 
+  <!-- Exclude RCPC2 from ARM64 R2R without asserting that the feature must be absent.
+       The explicit list requires its other features; keep other architectures conservative.
+       Resolve this after project/package imports have supplied their crossgen2 arguments. -->
+  <Target Name="_AndroidSetReadyToRunInstructionSet"
+      BeforeTargets="_PrepareForReadyToRunCompilation"
+      Condition=" '$(AndroidApplication)' == 'true' and '$(PublishReadyToRun)' == 'true' and '$(_AndroidUseOptimisticReadyToRunInstructionSet)' != 'true' ">
+    <PropertyGroup Condition=" !$(PublishReadyToRunCrossgen2ExtraArgs.Contains('--instruction-set')) and !$(PublishReadyToRunCrossgen2CompositeExtraArgs.Contains('--instruction-set')) ">
+      <PublishReadyToRunCrossgen2ExtraArgs Condition=" '$(RuntimeIdentifier)' == 'android-arm64' ">$(PublishReadyToRunCrossgen2ExtraArgs);--instruction-set:-optimistic,aes,crc,dotprod,lse,rcpc,rdma,sha1,sha2</PublishReadyToRunCrossgen2ExtraArgs>
+      <PublishReadyToRunCrossgen2ExtraArgs Condition=" '$(RuntimeIdentifier)' != 'android-arm64' ">$(PublishReadyToRunCrossgen2ExtraArgs);--instruction-set:-optimistic</PublishReadyToRunCrossgen2ExtraArgs>
+    </PropertyGroup>
+  </Target>
+
   <!-- Default feature switches -->
   <ItemGroup>
     <RuntimeHostConfigurationOption Include="Microsoft.Android.Runtime.RuntimeFeature.IsMonoRuntime"

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest2.cs

@@ -24,6 +24,8 @@ namespace Xamarin.Android.Build.Tests
 	[Parallelizable (ParallelScope.Children)]
 	public partial class BuildTest2 : BaseTest
 	{
+		const string Arm64ReadyToRunInstructionSet = "--instruction-set:-optimistic,aes,crc,dotprod,lse,rcpc,rdma,sha1,sha2";
+
 		static object [] MarshalMethodsDefaultStatusSource = new object [] {
 			new object[] {
 				/* isRelease */              true,
@@ -75,8 +77,10 @@ public void BasicApplicationPublishReadyToRun ([Values] bool isComposite, [Value
 			proj.SetProperty ("AndroidEnableAssemblyCompression", "false");
 			proj.SetProperty ("PublishReadyToRunComposite", isComposite.ToString ());
 
-			var b = CreateApkBuilder ();
+			using var b = CreateApkBuilder ();
 			Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+			var expectedInstructionSet = rid == "android-arm64" ? Arm64ReadyToRunInstructionSet : "--instruction-set:-optimistic";
+			StringAssertEx.Contains (expectedInstructionSet, b.LastBuildOutput);
 
 			var assemblyName = proj.ProjectName;
 			var apk = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, rid, $"{proj.PackageName}-Signed.apk");